Head mounted display (HMD) devices are employed for displaying and viewing visual content from a visual display source. An HMD device is configured to be worn on a user's head. An HMD device typically has (1) a single small display optic located in front of one of the user's eyes (monocular HMD), or (2) two small display optics, with each one being located in front of each of the user's two eyes (bi-ocular HMD), for viewing a wide range of visual display content by a single user. A bi-ocular HMD allows for the possibility that the user may view visual content in 3-dimensions. The HMD devices that can currently be found in today's military, commercial, and consumer markets are primarily goggles/eyeglasses type devices that are worn the way a pair of goggles or eyeglasses are worn, or they are helmet-mounted devices that are attached to a helmet that is worn on the user's head. Additionally, the HMD devices that can currently be found in today's market primarily rely on three different technologies, and thus typically fall into three different categories: refractive; diffractive; and laser writer.
A first category of HMD devices currently found on the market is the refractive HMD. Refractive HMD's use the optical physics principle of refraction in order to transmit the projection of visual content from a visual display source to a user's eye. Refractive HMD's work by transmitting a projection of visual content from a display source through a light transmission medium, typically a transparent plastic such as acrylic, to produce a final coherent and often magnified image to the user's eye. The light transmission medium is essentially a lens or series of lenses that bend and magnify the light waves from the visual source as they enter and exit the transmission medium so as to form the magnified cohesive image, similar to the operation of a magnifying glass. This is the dominant methodology employed in most HMD's on the market today.
While the refractive HMD may be the dominant methodology used in the HMD market, it does have several drawbacks. The problem with such refractive HMD's is that, with the transmission medium typically being large blocks of heavy plastic located in the optical path of the HMD, this type of HMD is very heavy, bulky, and cumbersome for a user to wear on either his head or face. This limits the overall comfort for the user wearing such an HMD. In addition, such a bulkier fit for the user significantly limits the styling that may be applied to such a device. Furthermore, because the refractive lenses of refractive HMD's are often located in the user's direct field of view, creating a refractive HMD that gives a user adequate “see-through vision,” or the ability to simultaneously see the projected visual content and at the same time clearly see through the projected content to the real-world outside surrounding environment, a “mixed-reality” view, becomes very complicated. Another drawback of refractive HMD's is that they can often prevent a user from seeing anything other than the projected visual content or can severely limit a user's peripheral vision, which can ultimately leave the user feeling claustrophobic. A further drawback of refractive HMD's is that, for those commonly found in the consumer or commercial markets, they have a very limited field-of-view (FOV) angle, with the typical FOV being about 25-degrees and the high-end FOV being about 40-degrees. When trying to increase the FOV of refractive HMD's commonly found in the consumer and commercial markets above the typical FOV of 25-degrees, the cost and weight of the device increases dramatically, which can be a significant prohibitive factor in two already competitive markets. This situation is apparent in the military market where refractive HMD's with FOV's between 40-degrees and 120-degrees are much more common, however as previously stated, they are extremely heavy and very expensive.
A second category of HMD devices currently found on the market is the diffractive HMD, or more accurately, a hybrid refractive/diffractive HMD. Diffractive HMD's use the optical physics principle of diffraction and diffraction gratings as well as refraction in order to transmit the projection of visual content from a visual display source to a user's eye. With this type of HMD, the projection of the visual content is passed through both a transmission medium and a diffraction grating contained within one of the refractive transmission medium elements to produce a final coherent and often magnified image to the user's eye. The light waves from the projected visual content that are passing through the transmission medium ultimately pass through or are reflected from the diffraction grating, which serves to present a single coherent image to the user. Various drawbacks to such hybrid HMD systems include bulkiness, high power light source requirements, and a limited field of view. These all limit their utility for military and industrial applications as well as their appeal for consumer applications.
A third category of HMD devices currently found on the market is the laser-writer HMD. The laser-writer HMD uses a remote laser light engine, often including a triad of red, green, and blue lasers, and a set of laser writers to bend and beam the laser lights, according to an input visual display signal, into a coherent visual image. The lasers and laser writer are connected to a head mounted display unit by coherent fiber optic cable in order to transmit the images to the head mounted unit. The images are then projected from the coherent fiber optic cable onto the final viewing screen, typically a transparent lens in the HMD unit, for viewing by the user. One drawback associated with this type of HMD is that the coherent fiber optic cable employed for such a system is very expensive. Another downside to such HMD systems is that, as the image comes out of the fiber optic cable, the head unit will still need some type of refractive optic to magnify the image, which in turn translates to a limited FOV and increased weight of the head unit. Furthermore, another downside related to laser-writer HMD's becomes apparent when using such a system to view visual content in 3D. To do so, the HMD system would typically either beam two distinct images to the head unit at the same time over a single fiber optic cable, thus the head unit would incorporate a beam splitter to separate the two images for each eye, or the HMD system would employ a second laser system working simultaneously with the first laser system in order to produce the second image employed to deliver 3D visual content. In either case, this can become extremely expensive. An additional downside to the laser-writer HMD device is that the power consumption to run such a device is extremely high. Lastly, transmitting an image to the head mounted unit via fiber optic cables can be potentially problematic if care is not taken to observe minimum bend radius of the fiber optic cable. If the cable is bent at too tight a radius, this will result in significant signal losses.
None of the above three categories of HMD systems that are available today are capable of providing magnified coherent visual content for viewing by a user from a single device that is all at once inexpensive, lightweight, comfortable, and that can be considered a near-to-eye HMD device. Consequently, because of the shortcomings and problems associated with the three types of systems currently available, there is a need in the industry for a new type of HMD device that is fairly inexpensive, lightweight, compact, comfortable, and is a near-to-eye device.
Optical Path
As discussed above, in HMD devices available today, the optical path typically involves refractive optics that are ineffective, heavy, and/or bulky. In augmented reality systems, optical see-through head-mounted displays (“OST-HMD's”) have been one of the basic vehicles for combining a computer-generated virtual scene with the views of a real-world scene. Typically, through use of an optical combiner, an OST-HMD maintains a direct view of the physical world and optically superimposes computer-generated images onto the real scene. Compared with a video see-though approach, in which the real-world views are captured by cameras, the OST-HMD has the advantage of introducing minimal degradation to the real world scene or providing a more accurate view. Therefore an OST-HMD is typically preferred for applications where a non-blocked real-world view is critical.
Designing an OST-HMD that has a wide FOV, a low F-number (which, in optics is also referred to as the focal ratio and is the ratio of the focal length to the diameter of the entrance pupil), is compact, and is nonintrusive has been a great challenge. Designing such an OST-HMD has been especially difficult to achieve with a non-pupil forming system, wherein the light rays from the image that enter the eye are essentially parallel, so an eye does not need to be located at a particular location to see the image formed by the light rays. Such a non-pupil forming system is in contrast to pupil-forming systems wherein the light rays converge to a definite point in space, and if an eye is positioned in front of or behind this point the image will not be visible. The typical eyepiece structure of HMD's available today uses rotationally symmetric components that are limited in their ability to achieve a low F-number, large eye relief, and wide FOV. Many methods have been explored to achieve an HMD optical system which fulfills the above highly desirable characteristics. These methods include: applying catadioptric techniques (techniques involving both refractive and reflective optics); introducing new elements, such as aspherical surfaces, holographic optical components, and diffractive optical components; exploring new design principles, such as using projection optics to replace an eyepiece or microscope type lens system in a conventional HMD design; and introducing tilt and decenter, or even free-form surfaces (FFS). (see, e.g., H. Hoshi, et. al, “Off-axial HMD optical system consisting of aspherical surfaces without rotational symmetry,” SPIE Vol. 2653, 234 (1996); and S. Yamazaki, et al., “Thin wide-field-of-view HMD with free-form-surface prism and applications,” Proc. SPIE, Vol. 3639, 453 (1999).).
Among the different methods mentioned above, free-form surfaces demonstrate great promise in designing compact HMD systems. It is challenging, however, to design a free-form prism based OST-HMD offering a wide FOV, low F-number, and sufficient eye relief. Many attempts have been made to design HMD's using FFS's, particularly in designs based on a wedge-shaped prism (see U.S. Pat. Nos. 5,699,194; 5,701,202; 5,706,136; and D. Cheng, et al., “Design of a lightweight and wide field-of-view HMD system with free form surface prism,” Infrared and Laser Engineering, Vol. 36, 3 (2007).). For instance, Hoshi et al. presented an FFS prism offering an FOV of 34° and a thickness of 15 mm. Yamazaki et al. described a 51° OST-HMD design consisting of a FFS prism and an auxiliary lens attached to the FFS prism. More recently, Cakmakci et al. designed a 20° HMD system with one free-form reflecting surface which was based on rational radial basis function and a diffractive lens. (“Optimal local shape description for rotationally non-symmetric optical surface design and analysis,” Opt. Express 16, 1583-1589 (2008)). There are also several commercially available HMD products based on the FFS prism concept. For instance, Olympus released their Eye-Trek series of HMD's based on free-form prisms. Emagin carried Z800 with the optical module WFO5. Daeyang carried i-Visor FX series (GEOMC module, A3 prism) products. Rockwell Collins announced the ProView SL40 using the prism technology of OEM display optics.
Existing FFS-based designs have an exit pupil diameter that is typically in the range of 4 mm to 8 mm, with a FOV typically around 40-degrees or less. In the more recent designs, smaller micro-displays, typically around 0.6″, were adopted, which employ a focal length of around 21 mm to achieve a 40-degree FOV. The reduced focal length makes it very challenging to design a system with a large exit pupil, or the virtual aperture in an optical system. As a result, most of the designs compromise the exit pupil diameter. Thus, commercially available products on average reduce the exit pupil diameter to being within a range of about 3 mm to about 5 mm in order to maintain an F-number greater than 4. There are a few designs that achieve a larger exit pupil by introducing additional free-form elements or diffractive optical elements. For instance, Droessler and Fritz described the design of a high brightness OST-HMD system with an F-number as low as 1.7 by using two extra decentered lenses and applying one diffractive surface. (U.S. Pat. No. 6,147,807). The existing work in the field of optics and HMD's shows that it is extremely difficult to design an HMD having both a low F-number (indicating a high magnification ratio) and a wide FOV.
Accordingly, it would be an advance in the field of optical see-through head-mounted displays to provide a head-mounted display which has a wide FOV and low F-number while also providing a compact, light-weight, and nonintrusive form factor.
Structural Support of Optical Components
An optical path for a HMD is defined by various optical elements that are held in precision alignment. A problem is how to maintain precision alignment of the optical path without undue weight and bulk. Some optical systems try to accomplish optical alignment by integrating all of the optical surfaces into a single, monolithic element, which combines both the refractive and reflective optics for the system in a single optical element. This is typically done using lens surfaces combined with prismatic optics having low internal reflection losses. These systems are heavy and bulky, and have additional manufacturing and assembly complications. Other optical approaches utilize individual optical elements which must be accurately aligned, both radially and longitudinally. Another approach utilizes a light waveguide, such as for example optical fiber or rectangular waveguides, to try and control alignment errors, which method ultimately provides very small FOV's.
A practical HMD needs to be small and light for user comfort, as in a pair of eye glasses. Most eye glass frames are flexible and do not provide precision alignment suitable for a HMD. What is needed is a lightweight eye glass configuration that is capable of providing a precision alignment to an optical path, and to accommodate multiple industrial designs without affecting the support or alignment of the optical elements.
Micro-Display Mechanism
An HMD includes an optical path between a display and a user's eyes. Eye comfort and ease of use are of paramount concern with all HMD's. The optical path and its various design parameters are one aspect of achieving eye comfort. However, there are many factors affecting eye comfort beyond the optical elements themselves. Human factors are of great concern in proper HMD design. People's eyes vary greatly from one person to the next, and even in the same person, from eye to eye. This makes it desirable to build in additional adjustments to the HMD to facilitate maximum eye comfort. Two concerns are the accommodation for different users of varying focal points, or eye focus, and varying interpupillary distances, or the distance between the center of the pupils of a user's two eyes. Thus there is a need in HMD systems for the ability to make optical adjustments, both front to back and laterally, to account for differences among users in individual eye focal lengths and interpupillary distances. Typical existing adjustments tend to be quite bulky. What is needed is a very compact and light focusing mechanism that can fit into a lightweight HMD.
HMD Structure and Assembly Process
Among other functions, a HMD defines an optical path between a micro-display and a user's eyes. Optical components should be in precise registration with respect to one another in order to provide an acceptable image to the user. At the same time the resultant HMD should be compact and light to be acceptable to a user. The design should also be able to withstand a physical impact without adversely affecting the alignment of the optical components.